In a two-cycle engine, an exhaust port and scavenging port are opened on an inner wall which defines a piston sliding surface of a cylinder. When the piston slides on the inner wall of the cylinder, the piston ring expands somewhat radially out of the opening and catches on the peripheral edge, transversely provided relative to the piston stroke, of the opening so that a ring sound is produced as a noise. When the port opening is formed to be square, the piston ring is released quickly to expand out on either one of the upper and lower edges of the opening and to slide in contact and interfere with an entire part of the outer edge, and as a result a comparatively large shock sound is generated. Such a shock sound is repeated with the reciprocation of the piston, and continued with the increase of the number of revolutions so that the sound results in a noise. Particularly, in a two-cycle engine designed to have high performance, because the upper edge of the port is formed to be straight to rapidly discharge combusted gases and suck in a gaseous mixture, the generation of the above-mentioned noise is considerable.
As a countermeasure to such noise, if the upper and lower edges, transverse to piston strokes, of the port opening are machined to be arcuate so that the part contacting the piston ring may be gradually increased and decreased, the sliding contact sound should be reduced. Further, if the boundaries between the port opening and the inner wall of the cylinder are not edged, but are curved, the shock at the time of the sliding contact should be reduced as much as possible.
Conventional machining for arcuately bevelling the port opening edges is generally made by cutting. However, such machining is very difficult, particularly in a two-cycle engine which has a small volume. It is complicated and difficult to accurately machine the opening shape of a port in a small diameter cylinder. Such work is done partly manually and requires great skill. The resulting bevels of the opening edge are not uniform and therefore there are precision and mass-productivity problems. When machine cutting is used in consideration of the precision and mass-production, a large cutting allowance is required. Therefore the port opening shapes vary, and the critical port timing is likely to be improper. Further, when the opening edge is arcuately shaped by cutting or the like, burrs may be produced on the boundary of the arcuate concave part with the inner wall surface of the cylinder, and such burrs will have to be removed in a separate step from the cutting work.
As described above, in the conventional method by cutting or the like, regardless of the manual or machine operations, the cutting amount is so large as to be liable to deform the port opening, the bevelling work is difficult, and burrs must be removed, thereby increasing the number of steps, and efficient and effective mass-production and precision are hindered.
Therefore, instead of the above, it has been suggested to press a square port opening with a roller or the like and arcuately bevel it. However, it is difficult to attain a sufficient arcuate shape around the port opening in a cylinder fitted with a cast iron sleeve and to make a necessary and sufficient bevel with great precision. Because in order to make a sufficient bevel with only the pressing work, it would be necessary to repeat the pressing work many times. Further, if the working pressure is increased radically to accomplish the above with a minimum of steps, the port opening is likely to be deformed.
In view of the above-mentioned conventional problems in bevelling ports in cylinders, especially for two-cycle engines, the present invention has been made to effectively solve such problems.